Well, I'm usually not interested enough in the female characters to bother with them and work on their characterisation. Many of my favourite male characters were also relatively under-developed in canon, but I liked them enough to develop them further in my fanfics (I have a soft spot for side characters). With female characters, I prefer to ignore them. And I don't find het fic one bit hot, so I have strictly no interest in making the pairings more believable. Even with the very few canon het pairings I ship, I don't want to imagine/read any smut with them.

I guess too much bad het in canon, which never left any room for believable canon slash, made me defensive. I don't want to give het a chance. It's all over the place in canon, I don't need it in fandom. ;)